PROJECT SUMMARY/ABSTRACT Mutations in the Progranulin (PGRN) gene have been linked to two distinct neurodegenerative diseases, frontotemporal lobar degeneration (FTLD) and neuronal ceroid lipofuscinosis (NCL). Accumulating evidence suggests a critical role of PGRN in the lysosome. However how PGRN regulates lysosomal function and protects against neurodegeneration remains elusive. We have identified prosaposin (PSAP), the precursor of saposin peptides essential for lysosomal glycosphingolipid degradation, as a PGRN binding partner. We further showed that PGRN facilitates PSAP lysosomal trafficking from the extracellular space via the neuronal trafficking receptor sortilin. We found reduced neuronal levels of PSAP and saposins in PGRN deficient mice and in FTLD patients due to PGRN mutations. Moreover, we showed that PGRN forms a complex with PSAP and lysosomal proteases cathepsin B and D. Cathepsin D deficiency has been reported to cause FTLD related pathology and our preliminary studies showed that impaired PSAP function also leads to FTLD related phenotypes in mice. Thus we hypothesize that PGRN is critical for proper PSAP and cathepsin B and D functions and impaired PSAP and cathepsin functions is one key disease mechanism of FTLD-PGRN. To test this hypothesis, we propose three specific aims. In Aim1, we will examine the role of PGRN in regulating PSAP function. PSAP trafficking, processing and glycosphingolipid metabolism will be assayed in WT and PGRN-/- cells and tissues as well as control and FTLD-PGRN patient samples. In Aim2, we will determine the role of PGRN in cathepsin B and D trafficking and activation by examining WT and PGRN-/- cells and tissues as well as control and FTLD-PGRN patient samples. In Aim3, we will assay FTLD like phenotypes in mice with different levels of PSAP or cathepsin B or D to determine whether partial loss of PSAP or cathepsin function could contribute to FTLD disease progression. Furthermore, PSAP or cathepsin B or D will be overexpressed via adeno associated viruses (AAV) to determine whether PSAP or cathepsin overexpression can rescue phenotypes associated with PGRN loss in mice. In summary, these proposed studies will shed light on how PGRN regulates lysosomal function and provide novel insights into the disease mechanism of FTLD. We expect the results from our studies to facilitate therapeutic development for FTLD-PGRN as well other neurodegenerative diseases with a reported role of PGRN, such as Alzheimer?s disease.